Survival rates for many cancer types have improved significantly in the last few years. However, resistance to chemotherapy remains a major issue in the clinic. There has been remarkable progress in identifying molecular alterations and cell-intrinsic mechanisms that promote resistance to chemotherapy in cancer cells. However little is known about the mechanisms acquired by these cells to persist as a minimal residual disease (MRD). While it is generally known that the presence of an MRD highly correlates with relapse after therapy and poor prognosis, the mechanisms underlying the survival of the MRD after chemotherapy remain unclear. I am interested in uncovering the mechanisms used by cancer cells to develop resistance to chemotherapy in vivo. To study this problem, I am using a mouse model of Ph+ pre-B cell acute lymphoblastic leukemia (Ph+ ALL). This model is based on mouse pre-B cells expressing human BCR-ABL that generate a leukemic phenotype restricted to hematopoietic tissues upon injection into recipient mice. To uncover mediators of chemotherapy response, we performed an in vivo RNAi genetic screen, using a library of 1300 shRNAs. This library included genes that have been implicated in cell microenvironment processes and paracrine signaling. In order to validate the results from the screen, we selected candidates that showed the most significant reduction in hairpin representation upon treatment with front-line chemotherapies (cyclophosphamide or doxorubicin) when compared to the untreated sample. Using this approach, we have identified several candidate proteins implicated in cell microenvironmental processes and paracrine signaling originating from the action of the front-line chemotherapeutics, in vivo. To uncover the mechanisms utilized by these proteins to mediate therapy response, I propose the following specific aims: 1. Assess the role of candidate factors identified in a genetic screen in therapy response in a mouse model of Ph+ ALL. 2. Identify the basic mechanisms of drug resistance mediated by these candidate proteins. Investigate how downstream signaling from these candidates converge on signaling networks emanating from DNA damage. We believe that the results obtained from this screen will uncover uncharacterized cell intrinsic and extrinsic drug targets that, when inhibited, can potentiate the effects of front-line chemotherapeutics, like cyclophosphamide and doxorubucin.